Practical Skills Overview

Practical skills carry significant marks in 0620. Whether you sit Paper 5 (laboratory practical) or Paper 6 (alternative to practical), the assessed skills are identical. This page covers every skill the mark scheme tests.

The assessed skills

Skill	What it means	Paper 5/6 marks
Planning	Designing an experiment to test a hypothesis	Variable
Observation	Recording what you see, smell, or measure	Variable
Recording	Presenting data clearly in tables and measurements	Variable
Analysis	Processing data, plotting graphs, identifying patterns	Variable
Evaluation	Identifying errors, suggesting improvements	Variable

Planning experiments

When asked to plan an experiment, include:

Variables

• Independent variable: What you change (e.g. concentration of acid)
• Dependent variable: What you measure (e.g. time for reaction to complete)
• Control variables: What you keep the same (e.g. temperature, volume of acid, mass of solid)

Name the specific variables, not vague descriptions. “The concentration of HCl” is better than “the amount of acid.”

Method structure

1. State the apparatus needed (specific: “250 cm3 beaker”, not “a container”)
2. Describe the procedure step by step, with quantities
3. Explain how you measure the dependent variable
4. State what you keep constant and how
5. Say how many readings/repetitions to take
6. Describe how to make it a fair test

Safety

Name the specific hazard and the specific precaution:

• “HCl is corrosive — wear safety goggles” (not just “be careful”)
• “Hydrogen is flammable — keep away from flames”

Making observations

Describing observations correctly

Use precise chemical language:

Observation	Correct phrasing	Incorrect phrasing
Gas produced	Effervescence / bubbles of gas	”It fizzed”
Precipitate	[Colour] precipitate formed	”It went cloudy”
Colour change	Solution changed from [colour] to [colour]	“It changed colour”
Dissolving	Solid dissolved / disappeared	”It melted” (melting is a physical change)
Temperature	Temperature increased by X degrees	”It got hot”

Common observations to know

Reaction	Observation
Metal + acid	Effervescence (hydrogen gas), metal dissolves
Carbonate + acid	Effervescence (CO2), solid dissolves
Alkali + acid	Temperature rise (exothermic), no visible change usually
Precipitation	Precipitate forms immediately when solutions mixed
Thermal decomposition	Colour change, gas evolved on heating

Recording data

Tables

• Draw with a ruler
• Each column has a heading with a unit, separated by a forward slash: “Volume / cm3”
• Independent variable in the left column
• Values recorded to the appropriate precision (matching the measuring instrument)
• Include repeat readings and calculate a mean if appropriate

Precision of measuring instruments

Instrument	Precision	Record to
Measuring cylinder (100 cm3)	1 cm3	Nearest 1 cm3
Burette (50 cm3)	0.1 cm3	Nearest 0.05 cm3
Electronic balance	0.01 g	2 decimal places
Thermometer (standard)	1 C	Nearest 0.5 C
Stopwatch	0.01 s	Nearest 0.01 s

Record all values to the same number of decimal places in a column.

Drawing graphs

Rules for full marks

1. Axes: Independent variable on x-axis, dependent variable on y-axis
2. Labels: Both axes labelled with quantity and unit
3. Scale: Choose a scale that uses more than half the grid. Use simple divisions (1, 2, 5, 10, not 3 or 7)
4. Points: Plot accurately. Use neat crosses (x) or dots in circles
5. Line: Draw a smooth best-fit curve or straight line through the points, NOT dot-to-dot
6. Anomalies: Circle any anomalous point. Do not include it in the line of best fit

Reading from graphs

• Interpolation: Reading values within the range of data (reliable)
• Extrapolation: Extending the line beyond the data (less reliable)
• Use a ruler to read values. Show construction lines on the graph.

Analysing data

Calculating from graphs

• Gradient: Change in y / change in x. Use a large triangle (at least half the line length). Show the triangle on the graph. Include units.
• Rate from a graph: Rate = gradient of a tangent to the curve at a specific point, or rate = 1/time for a fixed event.

Identifying trends and patterns

• “As [independent variable] increases, [dependent variable] increases/decreases”
• State whether the relationship is proportional (straight line through origin), linear (straight line not through origin), or non-linear
• Quote data from the table/graph to support your statement

Evaluating experiments

Sources of error

Type	Example	Improvement
Heat loss	Energy escaping to surroundings in calorimetry	Use insulated cup, lid, stir
Parallax error	Reading meniscus at wrong angle	Read at eye level, bottom of meniscus
Timing error	Human reaction time starting/stopping stopwatch	Use data logging, repeat readings
Incomplete reaction	Not all solid dissolved before measuring	Allow more time, check no solid remains
Impure reagents	Contaminated solutions	Use fresh, labelled reagents

Improving reliability

• Take repeat readings and calculate a mean (ignoring anomalous results)
• Use more precise measuring instruments
• Control variables more carefully
• Use larger quantities to reduce percentage error

Improving accuracy

• Calibrate instruments
• Use instruments with finer divisions
• Eliminate systematic errors (zero errors on balances)

Apparatus selection

Know which apparatus to choose:

Task	Apparatus	Why
Measure 25.0 cm3 accurately	Pipette or burette	More precise than measuring cylinder
Measure approximate volume	Measuring cylinder	Good enough for non-critical volumes
Measure temperature change	Thermometer (0.5 C precision)	Standard precision
Time a reaction	Stopwatch	Better than clock
Collect gas	Gas syringe or over water	Gas syringe is more accurate
Heat gently	Water bath	More controlled than direct flame
Weigh accurately	Electronic balance (0.01 g)	More precise than beam balance

Common exam mistakes

1. Not specifying quantities: “Add some acid” loses marks. “Add 25 cm3 of 1 mol/dm3 HCl” earns marks.
2. Confusing accuracy and precision: Accuracy is how close to the true value. Precision is how consistent the readings are.
3. Drawing dot-to-dot graphs instead of best-fit lines: Always draw a smooth curve or straight line.
4. Not controlling variables: In planning questions, you must state what you keep constant and how.
5. Saying “human error”: This is too vague for marks. Specify the actual error (parallax, reaction time, heat loss).

Worked exam questions

A student investigates how temperature affects the rate of reaction between magnesium and hydrochloric acid. Describe the method, including the variables. [5 marks]

• Independent variable: temperature of the acid (e.g. 20 C, 30 C, 40 C, 50 C, 60 C) [1]
• Dependent variable: time for the magnesium to completely dissolve / volume of hydrogen collected in a set time [1]
• Control variables: concentration and volume of acid (e.g. 25 cm3 of 1 mol/dm3 HCl), mass and size of magnesium (e.g. 0.1 g of ribbon) [1]
• Heat the acid to the required temperature using a water bath before adding the magnesium [1]
• Start a stopwatch when magnesium is added; stop when it completely dissolves or measure gas volume at regular intervals [1]

A graph of volume of gas against time shows a curve that levels off. The student has one anomalous point above the curve. State what the student should do with this anomalous point. [1 mark]

• Circle the anomalous point and ignore it when drawing the line of best fit [1]